1. Field of the Invention
The present invention relates to a dielectric ceramic composition containing an oxide of a barium titanate family, and more particularly to a dielectric ceramic composition suitably used for materials constituting electronic components that operate in a microwave (a few GHz) region.
2. Description of Related Art
A dielectric ceramic composition has been widely used for an electronic circuit board, electronic components or the like. In recent years, as electronic equipments such as a mobile communication equipment typified by a portable telephone have been becoming more and more prevalent in the world and demand for high performance of such equipments has been enhanced, the use of dielectric ceramic composition is becoming increasingly necessary for RF (Radio Frequency) circuit components contained in such electronic equipments.
A dielectric ceramic composition used for radio frequency circuit components is desired to provide superiority in electrical properties such as dielectric characteristics and temperature dependence of the resonance frequency (electrostatic capacitance). In more detail, the following electrical properties are required for said dielectric ceramic composition. That is, for example, at 1 GHz, relative dielectric constant is about 18 to 30, Q value is as high as about 8000 or more (in other words, dielectric loss is small), and the temperature dependence of the resonance frequency satisfies NPO characteristics defined by the EIA standard while change of the resonance frequency is required to be within ±10 ppm/° C. over a temperature range from −25° C. to 85° C. centered on about 25° C. In addition, recently, the dielectric ceramic composition is in many cases sintered together with a metal wiring pattern to manufacture electronic components such as a multi-layer antenna (MLA) and a RF module, and further, demand for low-price electronic components has been growing. In order to meet such manufacture trend and address such demand, even in a case where a low-cost material such as silver (Ag: its melting point is about 961° C.) and cupper (Cu: its melting point is about 1083° C.) is used for formation of the metal wiring pattern, the dielectric ceramic composition has to be sintered at temperatures lower than melting point of such a low-cost material so as not to make the material melt.
It has been known that a dielectric ceramic composition having superior electrical properties and used for radio frequency circuit is realized by, for example, the material of the BaO—MgO—Ta2O5—MnO2 family such as Ba(Mg1/3, Ta2/3)O3 and Ba(Mn1/2, Ta1/2)O3, the material of the MgO—TiO2—CaO family such as (Mg0.95, Ca0.05)TiO3, and the material formed by substituting part of titanium (Ti) of BaTi4O9 for strontium (Sr) and then adding a predetermined amount of magnesium (Mg). However, all of those compositions include disadvantages. That is, those compositions have to be sintered at temperatures equal to or higher than 1250° C. and therefore, cannot be sintered together with the above-stated low-cost materials used for formation of metal wiring pattern.
In addition, although it is known that a material formed by adding tin (Sn), zirconium (Zr), boron (B), lithium (Li), etc., to the MgO—TiO2—CaO family can be employed as a dielectric ceramic composition used for high frequency circuit, which ceramic composition can be sintered at low temperatures, i.e., 900° C. to 1050° C., this composition is inferior in electrical properties, particularly, Q value. Furthermore, problems have been found in some compositions that can be sintered at low temperatures. That is, such compositions often include lead (Pb) and therefore, adversely affect the environment while deteriorating their electrical properties.
In consideration of those problems, a new dielectric ceramic composition has been conceived and is disclosed in Japanese Patent Application Laid-open No. 2000-281442, in which the dielectric ceramic composition is formed by substituting part of titanium (Ti) of BaTi4O9 for zirconium and adding a predetermined amount of zinc (Zn), boron, lithium, calcium (Ca), silicon (Si), silver and copper to said substituted BaTi4O9. However, the dielectric ceramic composition formed as described above cannot sufficiently satisfy the above-stated requirement for temperature dependence of the resonance frequency. In addition, since the dielectric ceramic composition includes a total of B2O3 and Li2CO3 over approximately 6 wt % and B2O3 enhances vitrification (amorphization) of compound consisting of each of the above-stated elements, lithium included in the composition in a glass state easily exhibits ion conduction, likely causing deterioration of electrical isolation necessary for the composition.
In addition to the above-mentioned problems, the dielectric ceramic composition disclosed in Japanese Patent Application Laid-open No. 2000-281442 indicates another problem. That is, the dielectric ceramic composition includes a large amount of B2O3 and Li2CO3, both of which are apt to react with water from the air, and therefore, exhibits low chemical durability, which property is unfavorable for reliability of dielectric ceramic composition.